When you are connected to our push server, it is done via websockets, the shittier your internet connection is, the more likely you lose that steady websocket and it falls back to older less efficient methods to try to keep the information going back and forth.  The browser maintains a list of messages it gets and should only display newer ids than what is has, but it sounds like somewhere between your connection going unstable (or disconnected/restarted) and it switching to polling, that some packets are lost and it loses its track of the message IDs (or they come and get processed in the wrong order, or no confirmation of messages), which results in a redisplay when it it tries to give you the messages.

This is something I will look into for those that have issues, but it will be a bit before we can pencil that in.